1. Field of the Invention
The present invention relates to an optical sensor module including an optical waveguide unit and a board unit mounted with an optical element.
2. Description of the Related Art
As shown in FIGS. 13A and 13B, an optical sensor module is produced by individually producing an optical waveguide unit W0 including an under-cladding layer 71, a core 72 and an over-cladding layer 73 provided in this order, and a board unit E0 including an optical element 82 mounted on a substrate 81, and then bonding the board unit E0 to an end portion of the optical waveguide unit W0 with an adhesive or the like with the core 72 of the optical waveguide unit W0 in alignment with the optical element 82 of the board unit E0. In FIGS. 13A and 13B, reference numerals 75 and 85 designate a base and a sealing resin, respectively.
The alignment between the core 72 of the optical waveguide unit W0 and the optical element 82 of the board unit E0 is generally achieved with the use of a self-aligning machine (see, for example, JP-A1-HEI5 (1993)-196831). In this self-aligning machine, the optical waveguide unit W0 is fixed to a fixed stage (not shown) and the board unit E0 is fixed to a movable stage (not shown) for the alignment. Where the optical element 82 is a light emitting element, an alignment position (at which the core 72 is properly aligned with the optical element 82) is determined, as shown in FIG. 13A, by changing the position of the board unit E0 relative to one end face (light inlet) 72a of the core 72 with light H1 being emitted from the light emitting element, monitoring the amount of light outputted from the other end face (light outlet) 72b of the core 72 through a lens portion 73b provided at a distal end of the over-cladding layer 73 (monitoring a photovoltaic voltage developed across a light receiving element 91 provided in the self-aligning machine), and then defining a position at which the light amount is maximum as the alignment position. Where the optical element 82 is a light receiving element, the alignment position is determined, as shown in FIG. 13B, by changing the position of the board unit E0 relative to the one end face 72a of the core 72 with a predetermined amount of light (light emitted from a light emitting element 92 provided in the self-aligning machine and transmitted through the lens portion 73b provided at the distal end of the over-cladding layer 73) H2 being inputted from the other end-face 72b of the core 72 and outputted through a tail end portion 73a of the over-cladding layer 73 from the one end face 72a of the core 72, monitoring the amount of light received by the light receiving element (monitoring a photovoltaic voltage), and defining a position at which the light amount is maximum as the alignment position.
The alignment utilizing the self-aligning machine is highly accurate, but unsuitable for mass production with the need for labor and time.
A conventional optical sensor module which permits easy alignment without the need for the aforementioned machine and labor is known (see Japanese Patent Application No. 2009-180723).
In the optical sensor module, as shown in plan in FIG. 14A and in perspective in FIG. 14B with its right end viewed from a right upper side, an optical waveguide unit W1 includes an under-cladding layer 41 having opposite side extension portions (upper and lower portions on a right side in FIG. 14A) which extend in a core axial direction (in a rightward direction in FIG. 14A) and are free from a core 42, and an over-cladding layer 43 having opposite side extension portions which extend in association with the extension portions of the under-cladding layer 41. Extension portions 44 defined by these extension portions respectively have board unit engaging vertical grooves (engaging portions) 44a provided in a pair in proper positions thereof relative to a light transmission face (one end face) 42a of the core 42 as extending thicknesswise of the optical waveguide unit W1. On the other hand, the board unit E1 includes engagement plate portions (to-be-engaged portions) 51a provided in left and right edge portions (laterally opposite edge portions) thereof to be brought into fitting engagement with the vertical grooves 44a. 
In the optical sensor module, the board unit E1 is coupled to the optical waveguide unit W1 with the engagement plate portions 51a of the board unit E1 in fitting engagement with the vertical grooves 44a of the optical waveguide unit W1. Here, the vertical grooves 44a are designed so as to be located in the proper positions with respect to the light transmission face 42a of the core 42, and the engagement plate portions 51a are designed so as to be located in proper positions with respect to an optical element 54. Therefore, the fitting engagement between the vertical grooves 44a and the engagement plate portions 51a permits self-alignment between the core 42 and the optical element 54. In FIGS. 14A and 14B, a reference character 45 designates a base, and a reference character 45a designates a through-hole provided in the base 45 for receiving the board unit E1. Further, a reference character 51 designates a shaped substrate having the engagement plate portions 51a, and a reference character 55 designates a sealing resin.
Thus, the optical sensor module permits self-alignment between the core 42 of the optical waveguide unit W1 and the optical element 54 of the board unit E1 without the aligning operation. This eliminates the need for the time-consuming aligning operation, permitting mass production of the optical sensor module at higher productivity.
However, the optical sensor module often suffers from significant variations in optical coupling loss occurring between the core 42 and the optical element 54. The optical waveguide unit W1 suffers from slight variations in a distance Ls between the pair of vertical grooves 44a thereof (a distance between bottoms 44b of the opposed vertical grooves 44a) (see FIG. 15A), and the board unit E1 suffers from slight variations in the overall length Lc thereof (a distance between side edges 51b of the engagement plate portions 51a provided on the opposite sides thereof) (see FIG. 15B) in the production process. Although a relationship Ls=Lc should be satisfied according to design specifications, an actual relationship is Ls>Lc or Ls<Lc due to the tolerances of the components in the production process. If Ls>Lc, as shown in FIG. 15C, the board unit E1 wobbles (see an arrow F in FIG. 15C), resulting in inaccurate alignment and hence greater variations in optical coupling loss. If Ls<Lc, as shown in FIG. 15(d), the board unit E1 warps outward (in a direction such that the optical element 54 is displaced away from the light transmission face 42a of the core 42) to increase the optical coupling loss or, conversely, warps inward (in a direction such that the optical element 54 is displaced toward the light transmission face 42a of the core 42) in an non-illustrated manner to reduce the optical, coupling loss (in most cases, the board unit E1 warps outward as shown in FIG. 15(d)). This results in greater variations in optical coupling loss. Because of the greater variations in optical coupling loss, the optical sensor module including the board unit E1 and the optical waveguide unit W1 engaged with each other has a room for improvement.